1. Field of the Invention
The present invention relates to a radio receiver apparatus, and in particular, to a radio receiver apparatus of an orthogonal detection type comprising a local oscillation means with an improved automatic frequency control arrangement.
2. Description of the Related Art
In general, there has been used a single superheterodyne system or a double superheterodyne system as a reception system for radio communications. However, the above-mentioned conventional heterodyne system requires a band-pass filter for removing an image frequency and another band-pass filter for removing signals of adjacent channels. As each of the above-mentioned band-pass filters, a mechanical filter utilizing a mechanical vibration characteristic of crystal or ceramic has been used. Such a mechanical filter is accompanied by such a problem that the size of the mechanical filter is relatively large, and it is relatively expensive. As a reception system for solving the above-mentioned problem, there has been used a quadrature or orthogonal detection system for performing a demodulation or detection process by respectively mixing a reception signal with two local oscillation signals which are orthogonal to each other, i.e., which is phase-shifted by 90 degrees one another, thereby converting the reception signal into two signals orthogonal to each other.
A receiver apparatus utilizing the quadrature detection system is disclosed in the document of, for example, the U.S. Pat. No. 4,814,715 issued to Kasperkovitz, and entitled "MIXER ARRANGEMENT FOR SUPPRESSION OF OSCILLATOR INTERFERENCE IN QUADRATURE DEMODULATORS".
FIG. 1 shows a radio receiver apparatus utilizing a conventional quadrature detection system disclosed in the above-mentioned U.S. Pat. No. 4,814,715, and the radio receiver apparatus will be described hereinafter.
A radio frequency signal (referred to as an RF signal hereinafter) received by an antenna is converted into two base-band signals each including a direct current signal which are orthogonal to each other by a quadrature RF-tuning device T. Thereafter, unnecessary signal components of the base-band signals are removed through low-pass filters LP and LP', and then the resulting signals are outputted as a signal I.sub.1 and a signal I.sub.2.
The quadrature RF-tuning device T is comprised of quadrature mixing stages M.sub.T0 and M'.sub.T0 and a tuning oscillator T.sub.0. A mixer arrangement M is comprised of first and second quadrature mixer stages M.sub.1 and M.sub.2 for mixing quadrature mixing signals supplied from an oscillator F.sub.0 with the signal I.sub.1 and the signal I.sub.2, respectively. Output signals of the first and second quadrature mixer stages M.sub.1 and M.sub.2 are added together in a superposition circuit S.sub.0. Then the resulting added signal is demodulated by a processing reproducing arrangement P. In order to suppress the possible occurrence of cross talk, leak, and DC offset of the oscillator F.sub.0, a feedback is effected by means of first and second synchronous detectors SD.sub.1 and SD.sub.2 and low-pass filters LP.sub.1 and LP.sub.2.
However, in the radio receiver apparatus utilizing the conventional orthogonal detection system as shown in FIG. 1, the oscillator F.sub.0 oscillates and generates a sine-wave signal as a second local oscillation signal in a manner as shown in FIG. 1. Further, in the first and second quadrature mixer stages M.sub.1 and M.sub.2, there is performed the process of mixing of the signal I.sub.1 and the signal I.sub.2, respectively, with the sine-wave signal generated by the oscillator F.sub.0. Such a circuit construction is relatively complicated, and has not been able to use a clock signal of a microcomputer as the second local oscillation signal. Furthermore, by mixing the reception signal with the output signal of the tuning oscillator T.sub.0, namely, the first local oscillation signal having a frequency approximately equal to the center frequency of the reception signal, the reception signal is converted directly into the base-band signals orthogonal to each other. Each of the base-band signals obtained through the conversion process has a DC component. In particular, when the oscillation signal of the tuning oscillator T.sub.0 has a frequency drift equal to a deviation of the reception signal due to influence of temperature or a further factor, a significant great DC component is caused in each of the signal I.sub.1 and the signal I.sub.2, respectively. Therefore, each of the circuits for processing the signal I.sub.1 and the signal I.sub.2 is required to be a DC amplifier which permits that a direct current flows therein. If the DC component is removed, a reception sensitivity of the radio receiver apparatus may be seriously deteriorated when the first local oscillation signal has a frequency drift. However, such a DC amplifier circuit has been had such a problem that the DC amplifier circuit can not achieve a great amplification degree due to a drift of its reference point attributed to a change of temperature and a fluctuation of a power voltage. Furthermore, it is also possible to interrupt the direct current by means of a capacitor having a sufficiently great capacitance. However, the insertion of the capacitor has been accompanied by such a problem that it takes a long time from a timing when a power starts to be supplied to the receiver apparatus circuit to a timing when the receiver apparatus circuit becomes stable. Furthermore, there has been such a problem that a great noise component is generated at around the DC level due to 1/f-noise of the circuit, resulting in deterioration in the reception sensitivity.
FIG. 10 shows a local oscillator circuit of a prior art for a frequency modulation (FM) radio receiver apparatus.
Referring to FIG. 10, the local oscillator circuit comprises a time constant circuit 101 and a local oscillator 102. A demodulation signal outputted from a frequency to voltage converter (referred to as an f/V converter hereinafter) 100 functioning as an FM demodulator is inputted to the local oscillator 102 through the time constant circuit 101 comprising a in-series-connected resistance R and a in-parallel-connected capacitor C, and then the demodulation signal is passed through a circuit including a variable capacitance diode VD and a coupling capacitor Cc to a voltage controlled oscillator 103.
In the local oscillator circuit shown in FIG. 10, when the time constant of the time constant circuit 101 is set to a relatively small value, the frequency of the output signal of the local oscillator 103 is deviated or fluctuated due to a relatively low frequency component of the demodulation signal. On the other hand, when the time constant of the time constant circuit 101 is set to a relatively large value, it is possible to reduce of the deviation or fluctuation of the frequency of the local oscillator 103, however, there is such a problem that it takes a long time to complete an automatic frequency control (referred to as an AFC hereinafter) operation from a timing when a power switch is turned on.